In response to environmental problems, such as reduction of carbon dioxide emissions, energy saving of vehicles, construction machines, agricultural machines, and the like, namely fuel efficiency, has been an urgent issue in recent years. Devices such as engines, transmissions, final reduction gears, compressors, hydraulic systems, and the like are strongly requested to contribute to energy saving. Accordingly, for lubricating oils used for these devices, there is a demand for further reduction in agitation loss and rotation loss rather than conventional lubricating oils.
One of the means for saving the fuel of transmissions and final reduction gears is to reduce the viscosity of lubricating oils. For example, among transmissions, vehicle automatic transmissions and continuously variable transmissions have a torque converter, a wet clutch, a gear bearing mechanism, an oil pump, a hydraulic control mechanism, and the like, and manual transmissions and final reduction gears have a gear bearing mechanism. By reducing the viscosity of the lubricating oils used in these transmissions, the agitation loss and rotational resistance of the torque converter, wet clutch, gear bearing mechanism, oil pump, and the like, are reduced, and the power transmission efficiency is enhanced, thereby improving the fuel consumption of vehicles.
Conventionally proposed lubricating oil compositions are those that are obtained by mixing mineral oil-based and/or synthetic oil-based lubricating base oils mixed with various additives, and that have both high fuel efficiency and sufficient durability of metal parts, such as gears and bearings (see, for example, Patent Literatures 1 and 2)